A secondary cell gradually deteriorates after manufacture, and degradation occurs in characteristics, such as a decrease in capacity or a rise in resistance. The degradation of the cell affects a system which uses the cell. When the degradation is great, the cell capacity or output characteristics, which the system requires, fail to be satisfied, and there is concern that the operation of the system stops. Thus, it is important to estimate the degradation of the cell. Specifically, it is desirable that the system, which uses the cell, be capable of detecting the life of the cell. It is known that the speed of degradation of the cell varies depending on the method of use of the cell (e.g. the temperature of the environment, the range of charge/discharge), and some methods of estimating the degradation of the cell have been proposed.
There is a demand for the development of a large-scale battery apparatus which can supply large power, as a future battery apparatus. For example, there is a large-scale battery apparatus having a battery capacity of MWh or several-ten MWh class, which utilizes cells with a cell capacity of about 100 Wh. In this case, such a design is needed that 10,000 to 100,000 cells are combined in series or in parallel, in accordance with the required output voltage or current capacity.
In order to diagnose the degradation of each of cell modules which are mounted in the above-described large-scale battery apparatus, it is necessary to calculate the state of charge (SOC) or internal resistance value of the cell module as characteristic values for degradation diagnosis. Conventionally, a control device, which is configured to execute mutual communication with each cell module for the operation of the battery apparatus, calculates the above-described characteristic values for degradation diagnosis, in addition to executing the mutual communication.
However, since the load on the calculation of characteristic values for the degradation diagnosis becomes greater in accordance with the number of cell modules that are targets of diagnosis, this load becomes a cause of a delay or error in mutual communications for operations. Thus, a problem arises with the maintenance of safety in operation.